In mobile radio systems, provision is often made for measuring the output power in order to regulate the output power on the basis of the measured output power. Particularly in the case of mobile radio systems which utilize a variable-amplitude transmission method, it is generally necessary to measure and regulate the output power. One typical example of a variable-amplitude transmission method is so-called OFDM (orthogonal frequency division multiplexing), which is used particularly in WLAN radio systems according to the IEEE 802.11a/h Standard (transmission frequency in the region of 5 GHz) or 802.11g Standard (transmission frequency in the region of 2.4 GHz). In this case, the frequency band available for the radio transmission is subdivided into orthogonal partial frequency bands, for example 52 partial frequency bands, between which the data traffic of a radio link is divided. In this case, the output power as a superposition of the power of all the partial frequency bands may fluctuate over a wide dynamic range (approximately 17 dB) on account of the interference of the partial frequency bands. However, even in mobile radio systems according to the EDGE or UMTS Standard, which use a higher-order phase modulation (PSK—phase shift keying), the amplitude of the output signal is variable. Power regulation based on power measurement enables the optimum output power for the respective operating state to be present without the power amplifier that drives the transmission antenna being overdriven.
The output power is generally measured on the basis of the output voltage. This is usually done by measuring the voltage swing at the output of the power amplifier driving the transmission antenna. What is disadvantageous about the measurement on the basis of the output voltage is that the output voltage is dependent on the load of the power amplifier, as a result of which the output power measured on the basis of the output voltage may differ significantly from the power actually present. In particular for the case where the actual load on the power amplifier, that is to say the antenna with an optional matching network, deviates from the typical load for example due to component variation, aging or temperature effects, the actual output power cannot be deduced solely on the basis of the measurement based on the output voltage. A mismatch on the output side is present in this case.
When regulating the output power on the basis of the output voltage, the function of the power amplifier can be considerably impaired. The case where the load impedance of the power amplifier is lower than the typical value is conceivable, by way of example. If, in this case, the output amplifier already supplies the maximum possible swing of the output current, the actual swing of the output voltage is nevertheless lower than the maximum possible swing of the output voltage that should be sought on the part of the power regulation. The power regulation reacts to the measured power information based on the output voltage in such a way that it attempts to increase the swing of the output current. This causes the output stage of the power amplifier to be driven at the limit, so that the output signal is distorted by the nonlinear operation of the output stage. In this case, therefore, the power regulation drives the power amplifier into limiting operation.
Conversely, if the load impedance of the power amplifier is higher than the typical value, the misinterpretation of the power information measured on the basis of the output voltage on the part of the power regulation has the effect that the regulated output power is too low. Thus, the risk for power regulation that is based only on the measurement of the output voltage is that either the regulated output power is too low or the output amplifier is operated at the limit as a result of the regulation.
The problem described above can be avoided by providing an external directional coupler between the power amplifier and the antenna. Such a directional coupler has two measurement outputs, the output voltage of a radio frequency detector at the first measurement output being proportional to the power transmitted to the antenna and the output voltage of a radio frequency detector at the second measurement output being proportional to the power reflected from the antenna. The power radiated on the part of the antenna can be determined from the difference between the two variables. What is disadvantageous about such a solution is that additional components such as an external directional coupler and one or two radio frequency detectors are required. This causes additional costs and enlargement of the transmitter.